Structural steel

Structural steel is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and strength. Structural steel shape, size, composition, strength, storage, etc, is regulated in most industrialised countries.          

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Common structural shapes

In most developed countries, the shapes available are set out in published standards, although a number of specialist and proprietary cross sections are also available.

• I-beam (I-shaped cross-section - in Britain these include Universal Beams (UB) and Universal Columns (UC); in Europe it includes the IPE, HE, HL, HD and other sections; in the US it includes Wide Flange (WF) and H sections)
• Z-Shape (half a flange in opposite directions)
• HSS-Shape (Hollow structural section also known as SHS (structural hollow section) and including square, rectangular, circular (pipe) and elliptical cross-sections)
• Angle (L-shaped cross-section)
• Channel (C-shaped cross-section)
• Tee (T-shaped cross-section)
• Rail profile (asymmetrical I-beam)
  • Railway rail
  • Vignoles rail
  • Flanged T rail
  • Grooved rail
• Bar, a piece of metal, rectangular cross sectioned (flat) and long, but not so wide so as to be called a sheet.
• Rod, a round or square and long piece of metal or wood, see also rebar.
• Plate, sheet metal thicker than 6 mm or 1/4 in.
• Open web steel joist

While many sections are made by hot or cold rolling, others are made by welding together flat or bent plates (for example, the largest circular hollow sections are made from flat plate bent into a circle and seam-welded).

Structural steels

Most industrialised countries prescribe a range of standard steel grades with different strengths, corrosion resistance and other properties.

Standard structural steels (Europe)

Most steels used throughout Europe are specified to comply with the European standard EN 10025. However, many national standards also remain in force.

Typical grades are described as 'S275J2' or 'S355K2W'. In these examples, 'S' denotes structural rather than engineering steel; 275 or 355 denotes the yield strength in newtons per square millimetre or the equivalent megapascals; J2 or K2 denotes the materials toughness by reference to Charpy impact test values; and the 'W' denotes weathering steel. Further letters can be used to designate normalised steel ('N' or 'NL'); quenched and tempered steel ('Q' or 'QL'); and thermomechanically rolled steel ('M' or 'ML').

The normal yield strength grades available are 195, 235, 275, 355, 420, and 460, although some grades are more commonly used than others e.g. in the UK, almost all structural steel is grades S275 and S355. Higher grades are available in quenched and tempered material (500, 550, 620, 690, 890 and 960 - although grades above 690 receive little if any use in construction at present).

Standard structural steels (USA)

Steels used for building construction in the US use standard alloys identified and specified by ASTM International. These steels have an alloy identification beginning with A and then two, three, or four numbers. The four-number AISI steel grades commonly used for mechanical engineering, machines, and vehicles are a completely different specification series.

The standard commonly used structural steels are: ^[1]

Carbon steels

• A36 - structural shapes and plate
• A53 - structural pipe and tubing
• A500 - structural pipe and tubing
• A501 - structural pipe and tubing
• A529 - structural shapes and plates

High strength low alloy steels

• A441 - structural shapes and plates
• A572 - structural shapes and plates
• A618 - structural pipe and tubing
• A992 - W shapes beams only

Corrosion resistant high strength low alloy steels

• A242 - structural shapes and plates
• A588 aka Cor-ten - structural shapes and plates

Quenched and tempered alloy steels

• A514 - structural shapes and plates
• A517 - boilers and pressure vessels

Steel vs. concrete

As raw material prices fluctuate, often so does building design. During times of lower steel prices, more steel and less concrete is used, and vice versa. Each set of vendors and users typically maintain national industry associations that advocate the use of its materials versus the other. However, both materials are really needed together. Concrete without steel reinforcement (usually ribbed round bars called Rebar) is not structurally sound. Steel on its own, without solid concrete floors, is likewise not a preferred building method.

While rebar is almost always steel, it is not considered a structural steel and is described separately in the Rebar and Reinforced concrete articles.

Critical, and melting temperatures of structural steel

The properties of steel vary widely, based on what alloying elements are in it, and, for steel with carbon as its only alloying element, how much carbon is present.

The critical temperature for steel starts at 900°C for pure iron, then, as more carbon is added, the temperature falls to a minimum 724°C for eutectic steel (steel with only .83% by weight of carbon in it). As 2.1% carbon (by mass) is approached, the critical temperature climbs back up, to 1130°C.

This is not to be confused with the critical temperature for a fluid. The term "critical temperature", when used in regard to steel, means the temperature that all of the carbon in a steel is transformed into an austenitic crystal structure. This is very important for heat-treating steels.

In order for a fireproofing product to qualify for a certification listing of structural steel, through a fire test, the critical temperature is set by the national standard, which governs the test. In Japan, this is below 400°C. In China, Europe and North America, it is set at ca. 540°C. The time it takes for the steel element that is being tested to reach the temperature set by the national standard determines the duration of the fire-resistance rating.

Melting Point of Carbon only Steels

The bare minimum temperature that any alloy of Steel begins to melt is 1130 °C. Steel never turns into a liquid below this temperature. Pure Iron ('Steel' with 0% Carbon) starts to melt at 1492 °C (2720 °F), and is completely liquid upon reaching 1539 °C (2802 °F). Steel with 2.1% Carbon by weight begins melting at 1130 °C (2066 °F), and is completely molten upon reaching 1315 °C (2400 °F). 'Steel' with more than 2.1% Carbon is no longer Steel, but is known as Cast iron.

http://www.msm.cam.ac.uk/phase-trans/images/FeC.gif

Fire protection with steel vs. competition

Structural steel requires external insulation in order to prevent the steel from absorbing enough energy to reach its critical temperature (see above). First, steel expands, when heated, and once enough energy has been absorbed, it softens and loses its structural integrity. Given enough energy, it can also melt. This is easily prevented through the use of fireproofing. Likewise, although concrete structures on their own are able to achieve fire-resistance ratings, concrete is also subject to severe spalling, especially with elevated moisture inside the concrete at the time it becomes exposed to fire. There is also fireproofing available for concrete but this is typically not used in buildings. Instead, it is used in traffic tunnels and locations where a hydrocarbon fire is likely to break out. Thus, steel and concrete compete against one another not only on the basis of the price per unit of mass but also on the basis of the pricing for the fireproofing that must be added in order to satisfy the passive fire protection requirements that are mandated through building codes. Common fireproofing methods for structural steel include intumescent, endothermic and plaster coatings.

See also

References

1. ^ Manual of Steel Construction, 8th Edition, 2nd revised printing, American Institute of Steel Construction, 1987, ch 1 page 1-5